Specifically language impaired (SLI) children have particular difficulty learning and using grammatical morphemes, especially those regarded as low in phonetic substance (Leonard, 1989; Leonard, McGregor, & Allen, 1992). Their difficulty with these forms may relate to a general limitation in resource capacity (Leonard et al., 1992). Because these markers may place heavy perceptual demand on SLI children, SLI children may have few resources left to discover these elements; grammatical functions and lace them in an appropriate morphological paradigm. Some SLI children, however, may possess the knowledge of such morphemes but fail to deploy it during the on-lone processing of speech input because of the same general resource limitations. The proposed project will examine the relation between SLI children's knowledge of low versus higher phonetic substance morphemes and the on-line processing of these inflections, with the focus being to determine whether phonetic substances affects morpheme processing. Subjects, all having normal-range nonverbal IQ, will include 15 SLI and 15 comprehension-matched language normal (LN) children. Subjects will complete a (a) morpheme comprehension task and (b) word recognition reaction time (RT) task in which they will listen for familiar nouns occurring in fixed positions within sentences controlled for vocabulary, length, and complexity. Two sentence conditions have been created to examine the hypotheses that during sentence processing SLI children (a) should fail to perceive low-phonetic-substance inflections (past tense/ed/3rd person/s/) and (b) should perceive a higher-phonetic- substance inflection (present/ing/). In the Inflection condition, target nouns appear immediately after an appropriately inflected verb. In the Stem condition, target nouns occur after a verb that is inappropriately uninflected. Subjects will press a response pad immediately upon word recognition. Word recognition RT and number of errors (misses, false alarms) are the dependent variables. Within subjects ANOVAs will be performed to test the primary hypotheses that (a) SLI subjects should not show an RT difference between INF and STEM for the low-phonetic substance markers but should show a difference for the higher-phonetic marker, and (b) LN subjects should show an RT difference both in inflection types. A power analysis will be run to assess the probability of making a Type II error. To assess the relation between subjects' morpheme knowledge and processing, Pearson Product Moment correlations will be computed using subjects' morpheme comprehension scores and RT difference scores (difference between INF and STEM) for each morpheme type (low, high).